1. Field of the Invention
The present invention relates to a thrust roller bearing used in an automatic transmission and a compressor in an automobile, as well as to a cage used therein.
2. Description of the Background Art
A thrust roller bearing is constituted of a needle roller, a cage and a roller bearing ring, and has such a structure that the needle roller and the roller bearing ring are in line contact. Accordingly, high load-carrying capacity and high rigidity can be obtained for a small bearing projected area. As such, the thrust roller bearing is suitable as a bearing used in a harsh condition of use, such as poor lubrication or high-speed rotation, and widely used as a bearing for an automatic transmission or a bearing for a compressor of an air-conditioner in an automobile, for example.
FIG. 15A shows a roller holding portion provided in a pocket for accommodating a conventional needle roller. FIG. 15A shows a single-row roller, while FIG. 15B shows a multiple-row roller. Referring to these figures, in a cage, a pocket 105 accommodating needle rollers 102, 102a, 102b has a length Lp slightly larger than a length L of the needle roller. A roller holding portion 105a is provided on a periphery of a window in parallel to a central axis of the pocket, and projects toward the needle roller side above the needle roller axis. Here, roller holding portion 105a is provided on an entire periphery of the window in parallel to the axis. In such an arrangement of the roller holding portion, opening area, that is, a gap is inevitably small, and performance of oil passage to the inside of the cage is not sufficient.
As such, in order to increase an amount of passage of a lubricating oil per unit time, a thrust roller bearing as shown in FIGS. 16A to 16C has been proposed (Japanese Patent Laying-Open No. 2002-70872). In FIGS. 16A to 16C, a thrust roller bearing 50 includes a needle roller 80 and two annular cages 60, 70. Two cages 60, 70 are arranged so as to sandwich the needle roller from above and below, and have a window 61 of the upper cage and a window 71 of the lower cage respectively. Here, windows have a length larger than the roller length in a radial direction respectively. Roller holding portions 64, 74 formed in such plurality of windows sandwich and hold a plurality of needle rollers 80. In two cages 60, 70, roller holding portions 64, 74 have a length la in the radial direction shorter than a roller length l. In addition, at least one of two cages 60, 70 is bent so that at least one of thickness t1 of a radially outer portion 62 (72) and thickness t2 of a radially inner portion 63 (73) of the roller holding portion is made smaller than thickness t0 of the roller holding portion.
In this manner, a gap is enlarged in at least one of the radially outer portion and the radially inner portion having a thickness smaller than the roller holding portion. Accordingly, flow-in performance or flow-out performance of the lubricating oil passing through the gap is improved, and an amount of lubricating oil passing through the bearing per unit time can be increased. In addition, blocking by the cage of passage of the lubricating oil does not tend to occur, and stay of the lubricating oil is prevented. Consequently, temperature increase in the oil can be suppressed, thereby improving durability of the bearing. In the example above, in two cages 60, 70, outer plate portions 62, 72 are superposed, innermost portions 67, 77 of inner plate portions 63, 73 are bent in such a direction that they are superposed, and innermost portion 67 of inner plate portion 63 is caulked. In this manner, two cages 60, 70 are integrally fixed.
Though not limited to the thrust roller bearing, falling of a needle roller must not occur in particular in the thrust roller bearing under a condition of use in which the thrust roller bearing is operated under high load and in high speed. If the length of the roller holding portion is made smaller in the above-described thrust roller bearing, however, it will be difficult to hold the needle roller in a stable manner and falling of the needle roller from the cage may take place, though probability is low. In other words, improvement in the amount of oil passage of the lubricating oil results in a possibility of occurrence of falling of the needle roller, though probability is low. Hence, development of a thrust roller bearing free from falling of a needle roller while securing the oil passage performance of the lubricating oil has been demanded.
In addition, in the thrust roller bearing described above, the roller is pressed against the outer diameter side by centrifugal force during rotation. Therefore, unless an end face on the radially outer side of the pocket of the cage has a hardness and a case depth higher than a prescribed level, abrasion may occur, which will lead to breakage. Though it is possible to form a hardened case in the cage with heat treatment such as quenching, deformation by heat may occur due to quenching or the like. Accordingly, in order to prevent deformation of the cage due to heat treatment, prevention of deformation due to heat treatment by forming a corrugated portion in a cage has been proposed (Japanese Patent Laying-Open No. 2000-097242).
Even with the cage having a shape for preventing deformation due to heat treatment described above, however, if the case depth is too large, the cage tends to be embrittled, and a crack or the like may be produced during working or in use, resulting in breakage. In particular, in order to increase the thickness of the hardened case from a worked face, a time period for treatment should be extended, leading to cost increase. Therefore, development of a thrust roller bearing having a cage capable of overcoming the above-described problems as well as such a cage has been desired.
Moreover, in the conventional thrust roller bearing, an end face of the roller is pressed against the wall surface of the window by the centrifugal force. Furthermore, because of the contact caused by rotation of the needle roller, abnormal abrasion (hereinafter, referred to as “drilling abrasion”) is caused in the window. In addition, bearing noise has been large. As the thrust needle bearing is mainly used in an automobile, in recent days when quietness within a car is demanded, lowering of the bearing noise in the thrust roller bearing has also been demanded.